Meeting my BF's BFF whose in love with him

I have the most awkward situation to look forward to today. My not really bf but guy I'm dating bought me tickets to a festival today... he said his sister is coming - great! He's been trying to make us meet for a minute now. But then said he's bff is coming too... and I already know the back story on them. They went to 3rd base but not all the way, he decided to stay friends but she wants more and will take it at any chance she gets. She's bringing her ex girlfriend. See how messy this got so quickly? Should I go or not... and if I do how the hell should I handle this??? I was thinking about being drunk all day but maybe not.